I have a '73 F-250 4-speed manual 4wd 390, with a dana spicer 60 rear end. I was having problems with the odometer reading about 1.75 niles in a 30 mile stretch, but the speedo seemed to be working, so I swapped out the speedo/odometer from a '76 F-150.. the odometer is working now, but everything is a little bit off.

In a 1 mile stretch I first marked the odometer and it read that I went 1 and 1/10 of a mile. Then I marked the seconds it took me to run a mile at 60 mph and it took about 70 seconds.

Think I need to change the speedo gear in the tranny. I am running Mud Claw 32X11.50R15LT tires. rear gear ratio is 4.10.

I used the calculator on the http://www.tciauto.com/tc/speedometer-gear-calculator/ site and I get 18.4742286 as the answer, so should I go with the 18 tooth or 19 tooth gear? Almost ready to order one of each and just see which one works better, but would rather know ahead of time so I only order the one I need... the one currently installed is a 17 tooth.
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Bruspopper said:

... should I go with the 18 tooth or 19 tooth gear? Almost ready to order one of each and just see which one works better, but would rather know ahead of time so I only order the one I need... the one currently installed is a 17 tooth.
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Hmm, sort of depends on whether you would rather be going a little slower than you think or faster than you think. The ticket averse side of me says go with 18

Just a quick follow-up, I did buy a set of 5 (16 - 20), put in the 18, still about 10 mph off, put in the 19 still off by about 5 mph... finally put in the 20 tooth gear...wheeew, now when my speedo says I am running at 60, my phone app says 58... close enuff for me... just confused about the calculation stating i needed 18.47 but had to go all the way to a 20

The tires are taller than the stock tires, and make the difference.